1. Field of the Invention
The invention relates to a car radio with an electronic anti-theft device, which device comprises means for electrically detecting the removal of the car radio from its build-in location, and by which the function of the car radio is blocked as long as the anti-theft deice is not deactivated after the removal.
2. Description of the Related Art
German Patent DE-PS 37 17 054, corresponding to U.S. Pat. No. 4,808,981, is known a car radio with an electronic anti-theft device, this device comprising a memory for a codeword, means for entering the codeword, as well as means for electrically detecting a removal of the car radio from its build-in location, means blocking the functioning of the car radio as long as the codeword is not entered after the removal. To create a purely electronic anti-theft device which can neither be outwitted with an additionally connected battery, nor requires the use of an electronic key activating the circuit arrangement, the known car radio comprises a circuit arrangement which measures the impedance on at least one connecting terminal of the car radio when the device is switched on, compares the measured impedance value to the stored impedance value established previously at the build-in location and, when different values occur, blocks the functioning of the car radio until the right codeword is entered. Preferably, the impedance measurement is made on at least one loudspeaker terminal, while a signal generated in an AC voltage generator is applied to the loudspeaker terminal via a switch to measure the impedance. The impedance value on the loudspeaker terminal is then determined on the basis of the measured voltage on the loudspeaker terminal and on the basis of the AC current flowing through the loudspeaker circuit.
The codeword is to be entered in this car radio after it is built in and, simultaneously, the impedance value present on the loudspeaker terminal is to be entered in the memory as a digital value. For this purpose, the impedance value determined with said circuit arrangement is applied via an A/D converter to a microprocessor and written in the memory by this microprocessor. When the car radio is put in operation again, the microprocessor compares the impedance value present on the loudspeaker terminal to the impedance value stored in the memory when the car radio was put in operation for the first time. If the two values match, the car radio starts operating. If the two values do not match, the functioning of the car radio is blocked and the user is requested to enter the codeword. In the case of an authorized removal of the car radio from the vehicle, the entry of the codeword thus makes it possible to have the car radio operate again even, for example, with different loudspeakers.
Apart from a substantial cost of circuitry for the impedance measurement, the storing of the measured impedance values and the comparison of a newly measured impedance value to the stored impedance value shows that this anti-theft device does not provide a satisfactory protective function either. In the case of unauthorized removal of the car radio, it is possible to circumvent the compulsion of entering the codeword if, prior to the renewed, this time unauthorized operation, for example, loudspeakers are connected whose electrical characteristic values correspond to those from the vehicle of the authorized user. Such a circumvention of the anti-theft device may be facilitated in that, for example, the impedance values of loudspeakers for car radios are standardized. In order to avoid such a circumvention of the anti-theft device, a high-resolution impedance value should be determined by means of an accordingly high-cost impedance measurement. However, this may lead to the fact that already existing different transfer resistances on the connecting terminal, lead to the anti-theft device also being triggered by the authorized user. This also makes it difficult for the authorized user to handle the car radio; more particularly, the handling is affected by variations of the physical parameters which are inestimable for him.
In this case, the car radio described in DE-PS 37 17 054 does not have an advantage compared with the car radio in which, basically, after each disconnection, for example, from the battery of the motor vehicle, the codeword is required to be entered. But also a blocking of the car radio, when the operating voltage from the battery of the motor vehicle is interrupted, is hardly advantageous despite its simple realization, because it is useless in the cases where the car radio is stolen with connected voltage supply. Furthermore, it has appeared that the blocking of the above type causes problems, for example, during so-called start or load dump operations. By storing the voltage in capacitors inside the car radio, it is also possible to slow down the activation of the anti-theft device, i.e., the blocking of the car radio, so that in certain cases, the activation can be disturbed or prevented or is disturbed, respectively.